Second harmonic generation (SHG) involves the conversion of optical waves having an initial wavelength to optical waves having a wavelength which is one-half of the initial wavelength. Common second harmonic generation is achieved by coupling a beam of input waves into a nonlinear optical material wherein wavelength conversion is achieved. Various types of nonlinear optical materials have been reported as suitable for second harmonic generation. Among these are crystalline solids (see, e.g., U.S. Pat. No. 4,909,964), liquid crystals (see, e.g., G. R. Meredith et al., "Characterization of Liquid Crystalline Polymers for Electro-Optic Applications" A.C.S. Symp. Ser., 1983, 233 (Nonlinear Opt. Prop. Org. Polym. Mater.), 109-33), and polymer glasses (see, e.g., K. D. Singer, et al., "Second Harmonic Generation in Poled Polymer Films", Appl. Phys. Lett. 49(5) 1986).
Generally, a glassy material is prepared for nonlinear optical applications such as second harmonic generation by poling, a process wherein the material is cooled while under an electric field from a temperature above its glass transition temperature (Tg) to a temperature below its Tg. Poling allows polar alignment within the glassy material while under an electric field above its Tg. This alignment is locked in by cooling below Tg. Reheating the material above its Tg in the absence of a field generally results in rapid loss of alignment and a corresponding rapid loss of the bulk nonlinear properties desired for applications such as second harmonic generation. The poling of glassy polymers can be restricted by the polymer backbone and/or spacer groups included within the polymer.
Recently certain nonpolymeric organic glasses have been reported in M. Eich et al., "Second Harmonic Generation in Poled Organic Monomeric Glasses", J. Opt. Soc. Am. B. 6(8) pp 1590-1597, 1989, as suitable for second harmonic generation. Reportedly, compared with the case of polymers with attached or doped nonlinear moieties, one can achieve higher densities of nonlinear active groups and hence possibly higher optical nonlinearities in the nonpolymeric organic glasses, since no polymer backbone and spacer groups are incorporated. However, the compounds used, (s)-2-N-.alpha.-(methylbenzylamino)-5-nitropyridine and 2-N-(cyclooctylamino)-5-nitropyridine, have relatively low Tgs (i.e., about 0.degree. C. and -10.degree. C. respectively). Inasmuch as these Tgs are too low for most practical applications, the utility of these materials is considered to be limited.